Devices for measuring the segment height of a bifocal line are known. The segment height is a distance from the bottom of a spectacle lens or socket to a dividing line between a first power of a lens, say for reading, separating a second power region, say for distance viewing. The location of the line is important for the comfort of a user of spectacles. If the line is too high, a user will be aware of the two power zones while viewing objects at a distance. If the line is too low, there will be an insufficient zone for observing objects using the lower lens region. Ophthalmic practitioners usually decide on a height on a lens where the segment line is desired. This distance is recorded and sent to a lens maker. A scale for use in this measurement is shown in U.S. Pat. No. 3,987,554 to Pastore.
In recent years, with the proliferation of computers and video display terminals, there has arisen a need for special multifocal lenses. Such lenses have three separate powers, typically including a first power for reading, a second power for viewing a computer screen or video terminal and a third power for distance viewing. One of the problems associated with such multifocal lenses is in locating and sizing the various power zones, particularly the second power zone. It is known that, in order to minimize distortion, the three power zones should be located with the intermediate power zone between higher and lower power zones. For example, it is known that a suitable arrangement would be distance viewing zone, intermediate power zone, reading zone, respectively, from top to bottom.
An object of the invention was to determine the location and size of a second power zone in a multi-focal lens, especially a power zone used for viewing a video display terminal.